A gradiometer is a device for measuring the difference between two signals, with an aim to facilitate rejection of common mode noise signals and improved reduction in errors that arise from misorientation of the measuring instrument.
The ability of a gradiometer to measure small magnetic field gradients in the presence of a large background magnetic field is determined by its ability to reject large common mode signals, and the intrinsic noise and drift performance of the field detectors employed. The first of these characteristics is called the common mode rejection ratio (CMRR). In conventional gradiometers the CMRR is determined by the precision with which the field sensors comprising the gradiometer are balanced. That is, the CMRR is determined by the relative sensitivities of the field sensors, and the accuracy to which the field sensors can be positioned in precisely parallel planes. This fundamental nexus between the precision with which conventional gradiometers can be manufactured and the ultimate CMRR performance has until now limited the CMRR to figures of the order of 10−4 in practical instruments.
Further, the DC and low frequency accuracy of state of the art gradiometers is limited by the DC and drift performance of the field detectors employed, and is also limited by the low frequency noise performance of the field detectors. Furthermore, the achievable noise performance can be dependent upon the magnitude of the background homogeneous field; being determined by microphonics which arise from vibrations causing randomly varying misalignment of the axes of symmetry of the two field sensors. For example, in the case of magnetic gradiometry two appropriate magnetic sensors are the flux gate and the superconducting quantum interference device (SQUID). Flux gates have considerable drift and noise in comparison to SQUIDs, but state of the art SQUID systems are incapable of providing data about the absolute value of the magnetic field or gradient.
The inability of state of the art SQUIDs to measure the absolute value of the magnetic field or gradient arises for the following reasons. The radio frequency (rf) SQUID is essentially a ring of superconducting material, the ring being interrupted by a Josephson Junction. When the superconducting ring is energised by an inductively coupled resonant rf-oscillator, tunnelling of electrons takes place at the junction and a periodic signal, being a function of flux through the ring, can be detected across the junction. The periodic signal is substantially a triangular waveform, usually having a period (ΔB) in the order of a nanotesla. Therefore, in order to yield a sensitivity in the femtotesla range, the SQUID is operated in a nulling bridge mode, or flux locked loop (FLL) mode. In this mode, magnetic flux is fed back to the SQUID so as to cause the output voltage to remain relatively constant. The feedback voltage, being proportional to the difference between the applied flux and the quiescent flux level, gives a highly accurate measurement of relative magnetic flux. The feedback voltage V can therefore be written asV=M(AeffB+u)  (1)where
M is a constant in a specific SQUID system;
Aeff is the effective area of the SQUID;
B is the applied magnetic field; and
u is the quiescent flux.
However, the quiescent flux u is unknown. Thus, conventional rf-SQUIDs provide only relative measurements of magnetic field, up to an offset (which can usually be held constant for the duration of the measurement procedure) which is an integer multiple of a field value determined by the sensitivity of the SQUID and the fundamental flux quantum. The SQUID, therefore, is not an absolute field detector and state of the art gradiometers cannot provide absolute value measurements of the gradient components.
A further problem arises in tensor gradiometers. The gradient of a field in 3 dimensions is a second rank tensor, and accordingly has 9 components. In the case of magnetic fields Maxwell's equations impose restrictions which imply that only 5 of the gradient components are independent. To determine the gradient tensor of a magnetic field it is therefore necessary to measure all 5 of these components. State of the art tensor gradiometers achieve this by deploying a suite of at least 5 field sensors or planar gradiometers, or a combination of field sensors and planar gradiometers.
However, a problem which arises from the use of a plurality of field sensors or gradiometers to measure the gradient components, is that the sensors or gradiometers distort the field in their neighbourhood. Consequently, each sensor or gradiometer adversely affects the measurement of the field by the other sensors or gradiometers which are deployed to measure other gradient components. Such a “cross-talk” effect limits the accuracy of the gradient measurement.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
Throughout this specification, the terms ‘superconducting material’, ‘superconducting device’ and the like are used to refer to a material or device which, in a certain state and at a certain temperature, is capable of exhibiting superconductivity. The use of such terms does not imply that the material or device exhibits superconductivity in all states or at all temperatures.